Polarization rotation devices have been used for various purposes in optical systems, especially in fiber optic communication, optical image processing, and sensor applications particularly with the use of phase conjugate mirrors. The capability of these devices is demonstrated in FIG. 1. A phase conjugate mirror is desirable in fiber optical systems as a result of the signal distortion caused by fiber birefringence. Phase conjugation, by interchanging the bases of the incident beam polarization, allows a signal to return through a system and experience the reverse distortion as opposed to additional distortion. Faraday rotation, or the Faraday Effect, is one known method for creating a phase conjugation mirror. The Faraday Effect allows for the realization of devices such as fiber optic isolators, circulators, and Faraday rotating mirrors.
The Faraday rotation is determined by the following expression: θ=VBL, where θ is the angle of polarization rotation after a single pass through the rotator, V is the Verdet constant, B is the applied or internal magnetic field strength and L is the length of the rotator. The Verdet constant is a property inherent to a particular material and is highly dependent on both temperature and wavelength. This limits the use of systems employing Faraday rotation over broad temperature and wavelength ranges. In most applications the rotator is used in the magnetic saturation region so as to avoid variations due to the magnetic field. The length also presents precision rotation problems, as it cannot be exactly controlled under manufacturing conditions. With the use of thin film techniques and growth methods, the variations are slight but still present.
The manufacturing tolerance as well as temperature and wavelength-dependent nature of conventional single crystals for polarization rotation limit the use of optical crystal devices in precision instruments as well as over broad temperature and wavelength ranges. These conventional means only provide the desired polarization rotation at a single wavelength and at a certain temperature, with that temperature and wavelength being dependent upon the manufacturing accuracy. Advances in optical communication, sensors, and image processing require broadband, multi-wavelength capacities such as WDM, CWDM, DWDM, in central offices and uncontrolled field environment. Therefore, there remains a need to develop optical assemblies with precise polarization rotation independent of other variables.